Pressure and floatation questions are among the most predictable in general science — why a ship floats but a nail sinks, why a needle pierces skin easily, whose principle explains the hydraulic brake, why water boils faster on a hill. The concepts are few and they repeat every year. This post covers pressure and its everyday consequences, Archimedes' principle and floatation, Pascal's law and hydraulic machines, atmospheric pressure, and surface tension, followed by 15 previous year questions.
Key Points (Quick Revision)
- Pressure = force ÷ area; the SI unit is the pascal (Pa), equal to one newton per square metre
- Archimedes' principle — upthrust equals the weight of fluid displaced
- A body floats if its density is less than that of the fluid; ice floats with about one-tenth above water
- Pascal's law — pressure applied to an enclosed fluid is transmitted equally in all directions; it explains the hydraulic brake and hydraulic lift
- Atmospheric pressure falls with altitude, which is why water boils at a lower temperature on a hill and why a barometer measures altitude
1. Pressure and why area matters
Pressure is force divided by area. The same force spread over a small area produces a large pressure, and over a large area produces a small one. Almost every question in this section is an application of that one sentence.
| Observation | Reason |
|---|---|
| A sharp needle pierces easily | Its tiny tip gives a very small area, so pressure is high |
| A knife cuts better when sharpened | Sharpening reduces the area of the edge |
| A camel walks on sand without sinking | Its broad feet spread the weight over a large area |
| A tractor has wide tyres | Large area reduces pressure on soft soil |
| School bags have broad straps | Broad straps lower the pressure on the shoulder |
The SI unit of pressure is the pascal, named after Blaise Pascal, and one pascal is one newton per square metre. Other units you may meet are the bar, the atmosphere and the torr.
In a liquid, pressure depends on depth and density, not on the shape of the container. This is why a dam is built thicker at the bottom, and why a deep-sea diver needs protection that a swimmer at the surface does not. The pressure at a point in a liquid acts equally in all directions.
2. Archimedes' principle and floatation
Archimedes' principle states that when a body is immersed wholly or partly in a fluid, it experiences an upward force equal to the weight of the fluid it displaces. That upward force is called upthrust or buoyant force.
This is why an object feels lighter in water. Its apparent weight is its real weight minus the upthrust, and the loss in weight is exactly equal to the weight of the displaced liquid.
Whether a body floats or sinks follows directly from comparing densities:
- Density of the body less than the fluid — it floats
- Density equal to the fluid — it stays suspended anywhere in the liquid
- Density greater than the fluid — it sinks
An iron nail sinks while an iron ship floats, and the reason is shape rather than material. The ship is hollow, so its average density including the enclosed air is less than that of water, and the large hull displaces a great weight of water.
Ice floats on water because ice is less dense than water — one of the few substances that expands on freezing. Roughly one-tenth of an iceberg stays above the surface and nine-tenths below, which is the classic question from this section. It also explains why a pond freezes from the top down, allowing fish to survive beneath.
A ship rides higher in sea water than in river water, because sea water is denser and so a smaller volume needs to be displaced for the same upthrust. The instrument used to measure the density or relative density of a liquid is the hydrometer, and the one used to test battery acid is a form of it. Lactometer is used to check the purity of milk on the same principle.
3. Pascal's law and hydraulic machines
Pascal's law states that pressure applied to an enclosed fluid is transmitted undiminished and equally in all directions throughout the fluid.
The practical value of this is force multiplication. Apply a small force on a small piston and the same pressure acts on a much larger piston, producing a much larger force. That is the working principle of:
- Hydraulic lift — used to raise vehicles in a service station
- Hydraulic brakes — used in cars, where light pedal pressure produces large braking force
- Hydraulic press — used to compress material
- Hydraulic jack — used to lift heavy loads
Questions here are usually one line: which principle does the hydraulic brake work on — the answer is Pascal's law, and the common distractor is Archimedes' principle. Keep the two separate by what they explain: Pascal's law is about transmitting pressure, Archimedes' principle is about upthrust on an immersed body.
Two related ideas appear alongside these. Bernoulli's principle says that where the speed of a fluid increases, its pressure decreases — this explains the lift on an aeroplane wing and the working of an atomiser or spray pump. Viscosity is the internal friction of a fluid, which is why honey flows more slowly than water and why viscosity decreases as a liquid is heated.
4. Atmospheric pressure and surface tension
The air above us has weight, and the pressure it exerts is atmospheric pressure. It is measured by a barometer, and the mercury barometer was invented by Evangelista Torricelli. At sea level, atmospheric pressure supports a column of about 76 cm of mercury, which is taken as one atmosphere.
Atmospheric pressure decreases as altitude increases, and a chain of standard questions follows from that single fact:
- Water boils at a lower temperature on a hill, because boiling occurs when vapour pressure equals the surrounding pressure — which is why cooking takes longer at high altitude
- A pressure cooker does the opposite: it raises the pressure inside, so water boils above 100°C and food cooks faster
- Nose bleeding can occur at high altitude, because the pressure outside falls below the pressure of blood in the vessels
- A fountain pen leaks in an aircraft as the cabin pressure drops
- A barometer is used as an altimeter to estimate height, and a sudden fall in the barometer reading indicates a storm, while a steady rise indicates fair weather
Surface tension is the property by which the surface of a liquid behaves like a stretched membrane, caused by the attraction between the liquid's own molecules. It explains why raindrops and soap bubbles are spherical, since a sphere has the least surface area for a given volume, why a needle can be made to float on water, and why insects can walk on a pond surface.
Two exam points go with it. Surface tension decreases when temperature rises and when a detergent or soap is added — which is exactly why hot soapy water cleans better, as it wets the cloth more thoroughly. And capillary action, the rise of liquid in a narrow tube, is a related effect that explains the rise of oil in a lamp wick and of water in the narrow spaces of soil.
5. Previous year questions
- What is the SI unit of pressure? — Pascal (Pa)
- Pressure is defined as force divided by what? — Area
- Which principle explains the upthrust on a body immersed in a fluid? — Archimedes' principle
- Upthrust is equal to the weight of what? — The fluid displaced by the body
- Why does an iron ship float while an iron nail sinks? — The ship is hollow, so its average density is less than water
- What fraction of an iceberg remains above the water surface? — About one-tenth
- On which principle does a hydraulic brake work? — Pascal's law
- Which instrument is used to measure atmospheric pressure? — Barometer
- Who invented the mercury barometer? — Evangelista Torricelli
- Why does water boil at a lower temperature on a mountain? — Because atmospheric pressure is lower there
- Why does food cook faster in a pressure cooker? — Higher pressure raises the boiling point of water
- Which instrument is used to measure the purity of milk? — Lactometer
- Why are raindrops spherical? — Because of surface tension
- What happens to surface tension when a detergent is added to water? — It decreases
- Which principle explains the lift on an aeroplane wing? — Bernoulli's principle
6. How to revise this topic
Begin with the definition, because everything else grows out of it. Pressure is force divided by area — once that is automatic, every needle, camel, tyre and strap question answers itself without recall.
Then separate the three principles by what each one explains. Archimedes is upthrust, Pascal is transmission of pressure, Bernoulli is speed against pressure. Options in this chapter almost always offer all three together, so knowing which phenomenon belongs to which name is where the marks are.
For floatation, revise by comparing densities rather than memorising cases. Less dense floats, denser sinks, and the ship, the iceberg and the sea-water question all follow from that comparison.
Keep the instruments in one short list — barometer, hydrometer, lactometer — with what each measures, since they are asked directly and are swapped in options. Finally, note the five or six atmospheric pressure consequences together, because a question about the hill, the cooker or the aircraft pen is the same question wearing different clothes.
7. Frequently Asked Questions
Why does a sharp needle pierce more easily than a blunt one?
Because pressure is force divided by area. The sharp tip has a very small area, so the same force produces a much higher pressure at the point of contact. The blunt needle spreads the same force over a larger area and so exerts less pressure.
Why does an iron ship float while an iron nail sinks?
Because floatation depends on average density, not on the material. A ship is hollow, so the air enclosed within makes its average density lower than that of water, and its large hull displaces a great weight of water. A solid nail has density greater than water and sinks.
What is the difference between Archimedes' principle and Pascal's law?
Archimedes' principle deals with the upthrust on a body immersed in a fluid, which equals the weight of the fluid displaced. Pascal's law deals with pressure applied to an enclosed fluid being transmitted equally in all directions, and it explains hydraulic brakes, lifts and presses.
Why does water boil at a lower temperature on a hill?
Because atmospheric pressure falls as altitude increases. A liquid boils when its vapour pressure equals the surrounding pressure, so at lower pressure that point is reached at a lower temperature. This is also why cooking takes longer at high altitude.
Why are raindrops and soap bubbles spherical?
Because of surface tension. The surface of a liquid behaves like a stretched membrane and tends to take the shape with the least surface area for a given volume, which is a sphere.
For more General Science topics in simple language, see our Gravitation and States of Matter posts, or browse the General Science section. Preparing for a specific exam? Start with the SSC CGL guide or join the 100 Hour GS Course.